One of the common indicators when treating wastewater is COD. Many people still do not know what COD in wastewater is and what it means. This index is used to measure wastewater concentration to determine wastewater pollution. Please read along Duc Binh Microbial Products learn more specifically about COD as well as some other related indicators through the article: Updated wastewater treatment knowledge: What is cod in wastewater?
1. Concept of wastewater measurement indicators
1.1 What is BOD in wastewater?
BOD is the abbreviation for Biochemical oxygen Demand. This is the oxygen content required to help microorganisms decompose all organic matter under standard conditions (temperature + time). It can be understood that BOD indicates the amount of organic matter decomposed in the amount of water collected.
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BOD helps determine the amount of decomposed organic matter in the water sample
When the biological decomposition process occurs, microorganisms in the water will use dissolved oxygen. Determining the dissolved oxygen content required for the decomposition of organic matter will help us evaluate the impact of wastewater on water sources. In other words, the BOD index means the amount of organic matter oxidized by microorganisms.
⫸ See more: What is BOD? Solution to reduce BOD index in wastewater treatment
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1.2 What is COD in wastewater?
COD stands for Chemical Oxygen Demand which is the minimum oxygen content required to decompose inorganic and organic compounds in water. If the BOD index only indicates the amount of oxygen needed to decompose organic matter, the COD index indicates the amount of oxygen needed to decompose all chemical compounds in water. The amount of oxygen needed for decomposition is collected entirely from the amount of oxygen dissolved in water.
What is COD in wastewater?
COD is an important index in assessing water pollution (including wastewater, surface water and domestic water). It represents the amount of organic matter present in water. If the COD content is high, it means the water source contains many organic compounds causing pollution.
1.3 What is the DO index in wastewater?
DO is the abbreviation for Dissolved Oxygen, which means the minimum amount of dissolved oxygen in water that must be present for many organisms in the water to breathe… DO is the result of dissolution in the atmosphere or from the photosynthesis of algae. The oxygen content in water is influenced by temperature, pressure, terrain… This DO concentration usually falls at 8 – 10 ppm.
If the DO concentration is low, the dissolved oxygen content in the water will also decrease. From there, it will cause all living things to weaken and die. Therefore, when assessing the pollution level of water sources or conducting wastewater treatment, analyzing the DO index is extremely necessary.
In short, high levels of chemical and biological oxygen lead to a decrease in DO concentration in water and then harm the organisms in the water as well as the water ecosystem. The factors that cause high BOD and COD indexes in water are mainly organic, domestic and chemical wastewater.
2. What effects does high COD content in wastewater have?
The latest research projects clearly state: COD content in wastewater greatly affects the quality of water sources, the living environment and human health. Specifically as follows:
Polluting water sources
High COD in wastewater means that organic waste in wastewater will consume a large amount of oxygen when decomposed. Therefore, the amount of dissolved oxygen in water drops seriously. This has a great impact on organisms living in water, especially aquatic species that need a lot of oxygen to survive.
On the other hand, high levels of organic matter in wastewater will reduce the clarity, pH, and alkalinity of the water. Therefore, the quality of water resources is reduced.
Causes an unpleasant odor
Organic substances in many wastes, once decomposed, will produce gases with an unpleasant fishy odor. This smell spreads in the air and affects the quality of human life such as:
- Hydrocarbon compounds: Compounds containing the elements carbon and hydrogen with a strong odor.
- Sulfur compounds: Contains sulfur elements, has a foul odor
- Nitrogen compounds: Compounds containing the element nitrogen with a very strong odor.
Arising of infectious diseases
A high COD index means there is a large amount of organic matter in wastewater. This is a favorable environment for pathogenic microorganisms to operate, proliferate, and spread disease to humans:
- Diarrhea: It is a common intestinal infectious disease today caused by many types of viruses, bacteria or parasites. When we have diarrhea, we will have symptoms: Diarrhea, abdominal pain, fever, vomiting
- Cholera: A particularly serious intestinal infectious disease caused by the bacterium Vibrio cholerae. Its symptoms are: Severe diarrhea, vomiting, fever, abdominal pain.
- Typhoid: Caused by Salmonella typhi bacteria with symptoms: Fatigue, loss of appetite, diarrhea, headache, high fever.
- Hepatitis A: It is an infectious gastrointestinal disease caused by the hepatitis A virus with symptoms: Fever, yellow skin, yellow eyes, fatigue, loss of appetite.
Obviously, the COD index in wastewater seriously affects the environment. The higher this index, the greater the danger. Therefore, finding solutions to reduce the COD index is necessary.
⫸ See more: What is microbial sludge? The role of microbial sludge in wastewater treatment
⫸ See more: Applications biotechnology in wastewater treatment? advantages and disadvantages
3. Some methods help determine BOD, COD, DO
indexes
Depending on each index, there will be corresponding determination measures, but when measuring the concentration of those indexes we need to note the following points:
- Measuring pH levels as well as osmotic conditions must be appropriate.
- Recognize substances that are toxic to living things.
- Do not ignore temperature conditions and nutrients.
DO index evaluation method
To determine the DO value (mg/l measurement unit), we proceed in one of the following two ways:
- Winkler method (chemistry).
- Dissolved Oxygen Electrode Measure
Dissolved oxygen electrode method to evaluate DO index
BOD index evaluation method
To determine the BOD index, people do the following: dissolve the test water sample with a little deionized and oxygen-saturated water and add a sufficient amount of micro-organisms. The next step is to measure the dissolved oxygen concentration, then use a tight lid and place it in the dark at a temperature of 20 degrees Celsius to prevent photosynthesis from taking place. After 5 days, measure the dissolved oxygen concentration again. The change in DO for the first time and the next time is determined as the BOD value to be found.
The BOD value of the test sample is subtracted from the BOD value of the control sample to change the error to help give the most accurate BOD results. But today, people have another way to measure BOD called the Oxitop BOD measuring bottle method, which is done by: placing the BOD measuring bottle in a cabinet at a temperature of 20 degrees Celsius for 5 days. BOD will be automatically measured if the temperature is at 20 degrees Celsius. Results will also be automatically recorded after 1 day.
Oxitop BOD measuring bottle method helps automatically measure BOD when the temperature reaches 20 degrees Celsius
Method for determining COD index
What is the method for determining COD in wastewater? Previously, people often used KMn04 to determine the COD index. Currently this method is no longer applied because the effectiveness is not as expected. The oxidizing agent K2Cr2O7 was chosen to replace KMnO4.
Potassium dichromate is considered to be quite effective, reasonably priced, easy to prepare and can completely decompose organic substances. The method of determining COD using Potassium dichromate gives results after only 3 hours and COD data are converted to BOD only when the number of experiments is enough to draw a reliable correlation coefficient. The unit of measurement is mg/l
4. What is the way to reduce COD in wastewater?
Use probiotics
Use microorganisms to reduce the chemical oxygen demand that affects organic COD. There are 2 stages carried out in this process:
● Treatment with aerobic microorganisms
If wastewater has a chemical oxygen demand < 3000 mg/L requires aerobic treatment. COD in wastewater will be reduced by aerobic microorganisms that oxidize all organic matter. They belong to the group of heterotrophic microorganisms, using organic matter in wastewater as bait and then dividing to create new microorganisms. The process of decomposing organic matter continues.
Aerobic microbiological treatment for wastewater with chemical oxygen demand < 3000 mg/L
The main target of this measure is microorganisms, so providing good microbial strains is necessary.
● Anaerobic treatment
This method is suitable for wastewater with COD content > 2000 mg/L. Anaerobic treatment means a treatment environment without oxygen, used for anaerobic tanks such as UASB. This process is suitable with wastewater containing many organic substances. Supplying anaerobic microorganisms AD – Bot helps increase the efficiency of anaerobic treatment in wastewater treatment systems.
Using oxidizing chemicals
How to reduce the COD index by using oxidizing chemicals suitable for wastewater with little organic content and a lot of non-biodegradable substances such as phenol… So, which chemicals help reduce COD in wastewater? These are chlorine, hydrogen peroxide, ozone… but the dosage needs to be limited because they have the potential to harm wastewater treatment microorganisms.
Oxidizers are used for wastewater containing little organic matter and many non-degradable substances
Applying the Fenton reaction helps reduce COD
This reaction uses an oxidant to destroy pollutants. H2O2 + FeSO4 will produce hydroxyl free radicals that destroy all organic substances. When this reaction is complete, some organic substances will be converted to CO2 and water.
AOP Technology
This is the most advanced technology to help reduce wastewater COD (also for COD > 100000. The Fenton reaction mentioned above is the precursor of AOP technology in the presence of ozone. Therefore, providing additional free hydroxyl has the ability to oxidize chemicals in water.
New AOP technology that helps reduce the amount of COD in wastewater has many outstanding advantages
AOP technology has extremely fast response speed. While using microorganisms takes several days to produce results, AOP technology only takes a few hours and sometimes only a few minutes. Another advantage of this process is that it does not consume as much space as some other technologies. In addition, this technology does not require many chemicals.
Filter and absorb with activated carbon
This method is performed in the final stage or after the primary treatment process. Activated carbon can absorb organic substances, ozone or chlorine remaining in water. This will help keep the water completely cleaner and safer. In the water treatment process, when it is necessary to deodorize and reduce the amount of chemicals, people will also use activated carbon filtration.
⫸ See more: Uses and how to use activated carbon aquarium water filter
Using flocculation chemicals
Most COD comes from TSS or insoluble solids. We can use chemical coagulants (including PAC, alum and iron) and let these solids precipitate together to create a large block of sludge. Due to mass and gravity, the sludge settles in the primary sedimentation tank.
The method of using coagulation chemicals helps effectively reduce the amount of COD in wastewater
This sedimentation process affects COD concentration, especially in wastewater environments with high TSS levels. We need to pay attention to the mixing and sedimentation process so that the process of reducing wastewater COD can take place better. Stirring will help the precipitation reaction take place effectively.
Hopefully with the above information, readers have understood what BOD, DO, COD in wastewater are as well as ways to determine the index and ways to reduce COD. For specific advice and to provide the best microbial products for wastewater treatment, please contact Duc Binh Microbial Products – Center for Research and Application of Biological Products at:
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